(1) Field of the Invention
This invention relates to vibration dampers for use on cables suspended overhead and in particular to overhead electrical transmission cables.
(2) Description of Prior Art
Electrical transmission cables suspended overhead from towers are subject to a wind induced vibration identified as aeolian vibration. Any particular resonant vibration that may occur in an overhead conductor span is a function of a number of variables, among which are wind velocity and conductor tension. A typical overhead conductor for high voltage transmission of electrical power is comprised of a multi-strand conductor. The greater the tension on such a conductor, the less is the cost thereof since there is less sag and a consequent saving in material. The greater the tension, however, the greater is the propensity of the conductor to vibrate from the wind, and the less is the natural damping effect of the conductor itself because of lesser interstrand friction between the individual strands comprising the conductor. Even though the amplitude or intensity of aeolian vibration is small (rarely more than the diameter of the conductor when measured from peak to peak), the effect of such a vibration is to cause the conductor strands to fail in fatigue. One method of combating aeolian vibration is by the use of damping devices, an early one being the well-known Stockbridge damper. The Stockbridge damper is comprised of a symmetrical arrangement of weights and messenger cable clamped to the conductor. The function of the Stockbridge damper is to transform the vibratory motion of the conductor into another form of energy, namely, heat from friction between the strands of the messenger cable, and dissipate said heat to the atmosphere. Since the first Stockbridge damper, a vast number of damper devices have been developed in an attempt to effectively perform over a broad range of vibratory frequencies and other variables such as weather conditions. In most prior devices, the vibratory motion is transformed into heat either by friction, impact or a combination of the two. For example, U.S. Pat. No. 2,132,319 to Preiswerk discloses a damper comprised of a weight resiliently connected to a conductor, and damping is achieved by impacting the weight against stops mounted in fixed relation to the conductor.
Another U.S. Pat. No. (Re. 26,602) to Taylor teaches a damper comprising a weight connected by a flexible coupling to a support carried by the conductor, and vibratory motion of the conductor imparts a helical motion to the weight through flexible couplings, and energy is dissipated through flexing of ribs in the coupling.
U.S. Pat. No. 3,614,291 to Pullen discloses a damper which dissipates energy through a fluid containing dashpot.
Hawkins et al U.S. Pat. No. 4,110,553, issued Aug. 29, 1978, discloses a damper that utilizes friction of adjacent coils of a closely wound helical spring or springs to dissipate vibratory motion from an overhead conductor.
The aforementioned dampers are but a few of the many approaches in the art of damping vibratory cable motion.